Pitl, a tissue specific POU domain transcription factor that serves as a determining factor for three anterior pituitary cell types, was identified under this Award. Pit 1 regulates expression of many target genes, including growth hormone, prolactin, TSHalpha, and growth factor receptors including the growth hormone releasing factor receptor. This application proposes the definition of the molecular mechanisms by which the Pit 1 gene is initially activated and later exhibits auto regulation. Genetic, transgenic, and biochemical approaches will be used. The proposed studies include positional cloning of an informative regulatory locus and exploration of a novel pituitary HLH protein. The question of pituitary cell lineage will be investigated using contemporary gene rearrangement and transgenic technologies. Pit 1 acts in concert with other factors to determine cell specific patterns of gene expression, and the role of two novel homeodomain factors, Zn 15, and specific nuclear receptors in mediating the cell specific patterns of distal target gene expression will be studied. The possibility that novel co regulators of Pit 1 associate with distinct synergy domains in a gene specific fashion will be investigated. Current experiments that explore the role of the novel transcription factors in early pituitary development will be completed.